The properties recently required of the aqueous ink for aqueous inkjet printers are that the printed image has good fastness such as water resistance and light fastness, troubles such as irregular flow of aqueous ink or spreading of ink to enlarge the attached small ink droplet (hereinafter this is referred to as “blurring”) do not occur irrespective of the species of printing medium, and the printed image exhibits high density/color reproducibility irrespective of the species of printing medium.
Among these requirements, for ensuring the fastness of the printed image, studies are being made to use a pigment having excellent fastness in place of a dye. The pigment has no solubility in water unlike dye and therefore, needs to be dispersed in a fine particle state in water, but it is very difficult to stably maintain this dispersed state. To solve this problem, various techniques for stably dispersing a pigment in water have been proposed, such as a method of using a dispersible surfactant described in JP-A-1-301760 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and a method of using a dispersing polymer having a hydrophobic moiety and a hydrophilic moiety described in JP-B-5-64724 (the term “JP-B” as used herein means an “examined Japanese patent publication”). Also, with regard to the aqueous ink for aqueous inkjet printers, a technique of encapsulating the colorant surface with a polymer has been proposed, such as a method of using a microcapsule enclosing a dye aqueous ink described in JP-A-62-95366, a method of using a microcapsulated dye obtained by dissolving or dispersing a dye in a water-insoluble solvent and emulsifying the resulting solution or dispersion in water with a dispersible surfactant described in JP-A-1-170672, a method of preparing a microcapsule containing an inclusion product obtained by dissolving or dispersing a sublimable disperse dye in at least one of water, a water-soluble solvent and a polyester and using the microcapsule in a recording solution described in JP-A-5-39447, an ink composition comprising a colored emulsification-polymerized particle and an aqueous material described in JP-A-6-313141, and a method of using a phase inversion emulsification reaction or acid precipitation described in JP-A-10-140065.
As for the realization of a printed image having good color reproducibility, JP-A-5-155006 and JP-A-10-52925 describe a method of forming an image by combining a yellow aqueous ink, a magenta aqueous ink, a cyan aqueous ink and a black aqueous ink each obtained by dispersing a specific pigment with a water-soluble resin, and JP-A-2001-354886 describes a method of forming an image by combining a yellow aqueous ink, a magenta aqueous ink, a cyan aqueous ink, a black aqueous ink, a green aqueous ink and a red aqueous ink each obtained by dispersing a pigment with a water-soluble resin.
However, dispersing elements obtained by conventional techniques are unstable and when a surfactant or a substance having a hydrophilic moiety and a hydrophobic moiety, such as glycol ether, is present, absorption or desorption readily takes place and the aqueous ink disadvantageously suffers from poor storage stability. In the preparation of a normal aqueous ink, a surfactant or a substance having a hydrophilic moiety and a hydrophobic moiety, such as glycol ether, is necessary so as to reduce the blurring on paper. If such a substance is not used, the aqueous ink insufficiently permeates into paper and this causes a problem that the species of paper is limited for performing uniform printing or the printed image readily decreases in the quality.
Furthermore, when an additive for use in the present invention (that is, an acetylene glycol, an acetylene alcohol, a silicon-base surfactant, a di(tri)ethylene glycol monobutyl ether, a (di)propylene glycol monobutyl ether, a 1,2-alkylene glycol or a mixture thereof) is used in conventional dispersing elements, the long-term storage stability cannot be obtained and due to poor re-solubility of the aqueous ink, the aqueous ink is readily dried to cause a problem of clogging, for example, at the nozzle of an aqueous inkjet head or at the pen tip of a writing tool.
Also, the pigment dispersed by using such a dispersant has a problem in that the residual dispersant remains in the aqueous ink system and the dispersant does not satisfactorily contribute to the dispersion but desorbs from the pigment to increase the viscosity. If the viscosity increases, the amount added of the coloring material such as pigment is limited and a sufficiently high image quality cannot be obtained particularly on plain paper.
The present invention has been made to solve these problems and an object of the present invention is to provide an aqueous ink containing a dispersing element capable of giving an aqueous ink ensured with less blurring and high color formation on plain paper, satisfactory color formation and fixing on special paper, and in the aqueous inkjet recording, excellent ejection stability.